With Towers Gone, Area May Be Vulnerable to Lightning

When the World Trade Center stood, lightning regularly struck its towers and was safely discharged to the ground. Now, scientists say, Lower Manhattan may be much more vulnerable to lightning. Meteorologists and engineers have been given new impetus to study how lightning behaves in urban areas and how best to protect city dwellers.

''There may be a dramatic difference in New York City's lightning strike pattern,'' said Richard T. Hasbrouck, a lightning safety consultant based in Truchas, N.M.

The issue received little attention after Sept. 11, in part because New York had so few storms this year. But on Aug. 2, when an unusually powerful thunderstorm struck, a 25-year-old Manhattan man was struck and killed by a bolt of lightning on the roof of a six-story apartment building on Broome Street, on the edge of Chinatown.

''My first thought was, that wouldn't have happened if the World Trade Center were still there,'' said Jack Buchsbaum, chief electrical engineer of the Port Authority of New York and New Jersey, which owned the twin towers.

''I've been in the building when lightning hit and you couldn't even tell,'' said Mr. Buchsbaum, who was in his office on the 74th floor of the north tower on Sept. 11. (He now works in Newark.) With a lightning rod and broadcast antenna adding 360 feet to its height, the north tower was believed to be hit more often, Mr. Buchsbaum said, ''but no one here was recording the strikes or counting.''

At Vaisala-GAI Inc., a consulting company in Tucson, Ariz., scientists pinpoint and map lightning strikes across the world, using satellites that pick up the radio signals that lightning generates.

Because of geography and weather patterns, some states are more prone to lightning than others, said Ron Holle, a Vaisala-GAI meteorologist. New York State has a moderate incidence of lightning, with an average of 3.8 strikes per square mile a year, compared with a high of 20 to 42 per square mile in Florida and a low of fewer than 2 in California.

But because there is little research on what factors determine precisely where each bolt of lightning strikes, Mr. Holle said, it is hard to predict exactly how the loss of the trade center may change the strike pattern in Lower Manhattan. He said a thorough analysis could take five years.

In most lightning strikes, a strong negative charge builds up in a cloud when the moist air inside it becomes unstable. ''A thundercloud is a big blob of negativity,'' said Mr. Hasbrouck, the safety consultant. The charge then begins to reach downward in 50-yard increments, a spark that scientists call a ''stepped leader.''

Meanwhile, the cloud's negativity induces a positive charge in the ground beneath it. When these opposite charges meet, they interact explosively in a lightning strike.

Sometimes this means that a tall building itself can touch off lightning in a storm -- an effect first documented by General Electric scientists who studied the Empire State Building from 1931 to 1941. Such lightning goes from ground to cloud, while ''natural'' lightning is cloud to ground.

Skyscrapers' ''upward-reaching lightning'' appears to the eye as tree-shaped, said Dr. Martin Uman, the author of several books on lightning and a professor of electrical engineering and computer science at the University of Florida. Skyscraper lightning's pattern has its ''trunk'' on the bottom, branches on top. In contrast, natural lightning is often shaped like a tree held upside down.

Tall buildings have a predictable chance of being struck, one that increases with height. In a region with a moderate incidence of lightning, a 1,400-foot building (the north tower of the trade center and its broadcast antenna), the average would typically be about 35 times a year, said Dr. Dave Rust, a physicist who studies storm electricity for the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory in Norman, Okla.

By touching such lightning, tall buildings can sometimes divert charges in thunderclouds that would otherwise lead to strikes elsewhere, Dr. Rust said.

Whether the trade center's absence may have played a role in the fatal lightning strike in Lower Manhattan on Aug. 2 is impossible to determine. Dr. Uman notes that lightning can reach out as far as 10 miles from a thundercloud. The Broome Street building, at the corner of Mott Street, is less than a mile from ground zero.

Still, Mr. Holle of Vaisala-GAI said, ''You're never safe outside in a thunderstorm, but the buildings and open spaces around the World Trade Center are certainly more likely to get hit now.''

But quantifying the changes will require further study -- and perhaps a few seasons with more thunderstorms. Engineers are also trying to determine whether a widely used system for detecting lightning is effective in monitoring strikes in urban areas dense with tall buildings.

At a reporter's request, Vaisala-GAI conducted a computer analysis of its own data on Manhattan lightning strikes in the four years before Sept. 11 and the 11 months since. But the analysis produced a puzzling result, said a spokeswoman, Nancy Roth.

No lightning strikes to the World Trade Center showed up on the map. Neither did strikes at the Empire State Building, whose management company says it is hit dozens of times a year.

''That means one of two things,'' Ms. Roth said. ''Either there is no pattern of strikes at these two tall buildings, which seems unlikely, or for some reason our system does not pick them up.''

Mr. Hasbrouck said a possible explanation is that the presence of a large number of skyscrapers may somehow alter the electromagnetic signal associated with a lightning flash.

''It's an unexpected phenomenon, which will be investigated,'' he said.

Correction: September 8, 2002, Sunday An article in Science Times on Tuesday reporting that the loss of the World Trade Center may have left Lower Manhattan more vulnerable to lightning misstated the height of the north tower. It was 1,368 feet; the broadcast antenna added about 360, for a total above 1,700, not 1,400.